1. Field of the Invention
The present invention relates to a variable focus liquid lens, and more particularly, to a variable focus liquid lens using electrowetting, in which at least one of first and second liquids contains a surfactant and an interfacial portion exists between the first and second liquids, thereby reducing driving voltage and minimizing miscibility of the two liquids.
2. Description of the Related Art
In general, electrowetting is a phenomenon in which electric charge at a meniscus is adjusted to vary tensile force of the meniscus. The electrowetting can be used to control micro-fluid and micro-particles in the fluid. Recently, studies have been actively conducted on products using the electrowetting. The electrowetting basically utilizes an electric field and thus the response time is short and a product may be driven with relatively low voltage, which in turn enables miniaturization. The electrowetting has been extensively studied and applied to the fields of liquid lenses, display devices, optical devices, and Micro-Electro Mechanical Systems (MEMS).
However, in the prior art, the electrowetting has not been thoroughly researched, and the studies have been conducted or the products have been developed on the premise that there is no change in interfacial energy between solid and liquid forms, and between liquid and gas forms, allowing only simple control subject to differences in potential.
FIG. 1 illustrates an embodiment of a conventional variable focus liquid lens using electrowetting. As shown in FIG. 1, the conventional variable focus liquid lens 20 includes a solid plate 25 composed of an insulation layer 24 having a certain thickness d and electrodes 26 formed underneath the insulation layer 24, a conductive droplet 22 placed on an upper surface of the solid plate 25, and a driving source 29 having one end electrically connected to the droplet 22 and the other end electrically connected to the electrodes 26 to provide potential difference between the droplet 22 and the electrode 26.
With the above constitution, when the conductive droplet 22 is dropped on the insulation layer 24 and then the driving voltage is applied by driving source 29 between the electrodes 26 and the droplet 22, the curvature radius (the solid line in FIG. 1) of the droplet in a constricted state with no charge is enlarged into the shape of the droplet 28 (the dotted line in FIG. 1) due to the potential difference occurred between the electrodes 26 and the droplet 22. That is, the outer dimension of droplet 22, 28 is changed to vary the focus distance of the light passing therethrough.
In general, the relationship of the contact angle of the solid plate and the interfacial energy can be expressed by Young's equation (Equation 1 below).γSL=γSG−γLG COS θ  Equation 1
In the above equation, γSL represents the interfacial energy between solid and liquid, γSG represents the interfacial energy between solid and gas, and γLG represents the interfacial energy between liquid and gas, and θ represents the contact angle.
In general, when electrolyte exists between the electrodes, a numerical expression of thermaldynamics depending on the voltage application can be explained by Lippman's Equation (Equation 2 below).
                    γ        =                              γ            0                    -                                    1              2                        ⁢                          cV              2                                                          Equation        ⁢                                  ⁢        2            
From Equations 1 and 2 above, Lippmann-Young Equation (Equation 3 below) is derived.
                              cos          ⁢                                          ⁢          θ                =                              cos            ⁢                                                  ⁢                          θ              0                                +                                    1                              γ                LG                                      ⁢                          1              2                        ⁢                          cV              2                                                          Equation        ⁢                                  ⁢        3            
In the above equation, θ represents the contact angle when the voltage is applied, θ0 represents the initial contact angle, c represents the electric capacity, and V represents the applied voltage.
The above Lippmann-Young Equation does not take account for the changes in the interfacial energy between the initial liquid and gas and between solid and gas. The conventional device using electrowetting depends only on the applied voltage, allowing only simple control according to the potential differences.
A modified form of the above Lippmann-Young Equation is as follows in Equation 4 below.
                              cos          ⁢                                          ⁢          θ                =                              cos            ⁢                                                  ⁢                          θ              0                                -                                    ɛ                              2                ·                                  γ                  1                                ·                d                                      ⁢                          V              2                                                          Equation        ⁢                                  ⁢        4            
In the above equation, θ represents the contact angle when the voltage is applied, θ0 represents the initial contact angle, ε represents the dielectric constant between the electrodes, d represents the thickness of the insulation layer, V represents the applied voltage, and γi represents interfacial energy between insulation liquid and electrolyte.
This equation is a general one explaining the operational characteristics of electrolyte and insulation liquid illustrated in FIG. 1. In the above equation, as the γi is smaller, the change in the contact angle of the insulation liquid becomes bigger, which is expected to result in reduction of the driving voltage. However, if the interfacial energy of the two liquids is too small, it is difficult for the two fluids (the electrolyte and the insulation liquid) to exist independently, which may result in mixing of the two fluids or turbidity. Furthermore, the conventional liquid lens using electrowetting cannot achieve a sufficiently stable state, that is, the applied voltage is too high for the surface of the droplet to remain stable and for the droplet to maintain a uniform shape.